TWI451973B - Light diffusion polyester film - Google Patents

Description

Light diffusing polyester film

The present invention relates to a light diffusing film used for a backlight unit, an illumination device, or the like of a liquid crystal display. More specifically, the light-diffusing polyester film which coexists light diffusing property and light transmittance, and does not curl at high temperature processing.

In recent years, the technological progress of liquid crystal displays has been surprisingly widely used as a display device for personal computers, televisions, mobile phones, and the like. In particular, in recent years, various uses of liquid crystal displays have progressed toward high definition, especially in television applications. With the popularization of high-resolution playback, the so-called horizontal 1920×vertical 1080 centered on the adoption of large-screen LCD TVs. The LCD panel with full HD display is also used in LCD TVs with relatively small screen sizes, and the demand for high definition is getting higher and higher. Since these liquid crystal displays do not have an illuminating function by themselves, the liquid crystal display unit has a backlight unit provided on the back surface thereof to be displayable.

In the backlight unit, a light source is provided directly under the light-emitting surface, or an edge-light type in which light is introduced from the outside of the light-emitting surface via a light guide plate. Further, a light diffusing film is provided on the backlight unit, and light is diffused and scattered to make the brightness of the illuminating surface uniform. Further, in order to increase the front luminance, in order to concentrate the light penetrating the light diffusing film in the front direction as much as possible, there is a case where a sheet having a condensing function called a lens sheet is used. On the surface of the sheet, a large number of minute irregularities such as a ridge shape, a wave shape, or a pyramid shape are arranged side by side so that the light emitted from the light diffusing film is refracted toward the front surface to increase the brightness of the illuminating surface. Such a lens sheet is used by disposing one or two sheets on the surface side of the light diffusing film. In addition, in order to make the luminance unevenness due to the arrangement of the lens sheets or the defects of the lens sheet inconspicuous (improving the concealability), a light diffusing film may be disposed on the surface side of the lens sheet.

In the light-diffusing film used for the backlight unit described above, a diffusion layer made of a transparent resin containing fine particles is applied to the surface of the base film (Patent Documents 1 and 2).

However, in this method, since the light diffusion layer must be provided by coating on one side of the base film, the light diffusing film becomes a double metal due to the difference in the linear expansion coefficient of the light diffusion layer and the base film. The structure has a problem that curling due to heating is liable to occur. This problem is becoming an important problem especially in a liquid crystal display using a direct type backlight unit, such as a large liquid crystal TV, which requires a large and extremely high brightness. This is because the larger the area of the light diffusing film is, the more significant the curl is. Further, the higher the brightness of the display, the greater the power consumption of the light source, that is, the greater the heat generated by the backlight unit.

On the other hand, in recent years, in order to reduce the number of components of the backlight unit, or to simplify the process and reduce the cost, it is also considered to integrate the light diffusing film with other optical functional films (Patent Documents 3 and 4). However, since the light diffusing property is imparted by the light scattering material inside the substrate, a part of the incident light system is backscattered, and there is a problem that the light transmittance is lowered.

In addition, in recent years, the biaxially stretched polyester film itself having excellent heat resistance, mechanical strength, and thickness uniformity has been studied to impart light diffusibility (Patent Document 5). However, it does not impair the characteristics (heat resistance, mechanical strength, etc.) originally possessed by the biaxially stretched polyester film, but it may impair the characteristics of the light diffusing film such as light transmittance or light diffusibility.

Further, a light-diffusing layer in which a polyester having a melting point of 210 ° C or less or an amorphous resin is used as a constituent resin and a non-coherent particle or a thermoplastic resin is blended in the constituent resin is disclosed. In the intermediate layer, a film of a crystalline polyester resin layer is laminated on both surfaces (see Patent Documents 6 to 13).

However, there is no change in the large crystallinity difference between the light diffusing intermediate layer and the surface layer, and there is a planarity in the case of temperature change due to variations in layer thickness or physical properties in the surface. Significantly worsened, the problem of reduced mechanical strength.

In view of the above problems, it is proposed to use a light diffusion layer mainly composed of a crystalline polyester, and to have excellent heat resistance and mechanical strength inherent to the biaxially stretched polyester film, and to impart light diffusibility mainly by surface haze. A light diffusing polyester film in which total light transmittance and light diffusibility coexist (Patent Documents 14 and 15).

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] JP-A-6-59108

[Patent Document 2] License No. 3698978

[Patent Document 3] JP-A-9-281310

[Patent Document 4] License No. 3732253

[Patent Document 5] JP-A-2005-181648

[Patent Document 6] JP-A-2001-324606

[Patent Document 7] JP-A-2002-162508

[Patent Document 8] JP-A-2002-182013

[Patent Document 9] JP-A-2002-196113

[Patent Document 10] JP-A-2002-372606

[Patent Document 11] JP-A-2004-219438

[Patent Document 12] JP-A-2004-354558

[Patent Document 13] JP-A-2004-354558

[Patent Document 14] JP-A-2009-48156

[Patent Document 15] JP-A-2009-139684

The light-diffusing polyester film disclosed in Patent Documents 14 and 15 has characteristics of excellent mechanical strength, light diffusibility, and high light transmittance.

A smooth surface may be provided on the opposite side of the light diffusing layer of the light diffusing polyester film, and a suitable lens layer may be provided to form a lens sheet. When a lens layer is provided on the light-diffusing polyester film, a mold member of a roll mold having a transfer lens shape is usually used, and a light-diffusing polyester film in which a lens resin such as an ultraviolet curable resin is laminated is passed thereon. A lens layer is formed while imparting active energy.

However, in recent years, as the productivity has increased, the speed at which the substrate film passes through the mold member of the roll mold is increasing. Therefore, since the productivity of lens processing is improved, a film which can be used even under processing conditions of a higher temperature (for example, 100 ° C or higher) is required.

Accordingly, an object of the present invention is to provide a light-diffusing polyester film which has excellent mechanical strength, light diffusibility, and high light transmittance, and is excellent in high-temperature workability.

In order to solve the above problems, the inventors of the present invention conducted a review of the intensive efforts, and as a result, it has been found that the above problem can be solved by a light diffusing polyester film having a layer structure of three layers each having a different function and structure laminated by a co-extrusion method. The invention is achieved. The light-diffusing polyester film of the present invention which achieves the above object has the following constitution.

That is, the first invention of the present invention is a light diffusing polyester film which is a light diffusing polyester film formed by a biaxially oriented polyester film; and has light diffusion on one side of the intermediate layer (A). The layer (B) is formed by a three-layer structure in which a smooth layer (C) is laminated on the opposite surface by a co-extrusion method; the intermediate layer (A) is composed of a crystalline homopolyester or a crystal containing a copolymerization component. The light-diffusing layer (B) is composed of 50 to 99 parts by mass of a crystalline polyester containing a copolymerization component having a melting point of 225 to 255 ° C and 1 to 50 parts by mass of incompatible with the polyester. The smooth layer (C) is formed of a crystalline polyester containing a copolymerization component having a melting point of 225 to 255 °C.

According to a second aspect of the present invention, in the light diffusing polyester film, the surface haze is 15% or more, and the internal haze is lower than the surface haze.

According to a third aspect of the present invention, in the light diffusing polyester film, the average slope (Δa) of the surface of the light-diffusing layer (B) is 0.03 or more.

According to a fourth aspect of the invention, the light diffusing polyester film has a dimensional change ratio at 150 ° C of 3% or less in the longitudinal direction and the transverse direction, and a tensile strength of 100 MPa or more in both the longitudinal direction and the transverse direction, and a tensile elongation of 100 Å. %the above.

According to a fifth aspect of the present invention, in the light-diffusing polyester film, a rectangular film sample having a length of 300 mm in the longitudinal direction and a width of 210 mm in the film is heat-treated in a heating oven at a temperature of 150 ° C for 30 minutes. The height of the curl is on average 3.0 mm or less.

According to a sixth aspect of the present invention, in the light diffusing polyester film, the total light transmittance is 86% or more, and the image brightness of the comb width of 2 mm is 40% or less.

According to a seventh aspect of the invention, there is provided the light diffusing polyester film, wherein the surface of the light diffusing layer (B) has a copolymerized polyester resin or a polyamine disposed before the film is stretched and aligned. A coating layer containing at least one of a formate resin or an acrylic resin as a main component.

According to an eighth aspect of the invention, the light diffusing polyester film of the light diffusing layer (B) and the smoothing layer (C) of the light diffusing polyester film has a copolymerization polymerization. A coating layer containing at least one of an ester resin, a polyurethane resin, or an acrylic resin as a main component.

According to a ninth aspect of the invention, the light diffusing polyester film for a lens sheet is provided on the surface of the smoothing layer (C) of the light diffusing polyester film to copolymerize a polyester resin or a polyurethane. A coating layer containing at least one of an ester resin or an acrylic resin as a main component.

The light-diffusing polyester film of the present invention has the excellent mechanical properties inherently of the biaxially stretched polyester film, and has no effect of heating due to the coexistence of total light transmittance and light diffusibility, and achieves good results. High temperature processability.

Form of implementing the invention

The light-diffusing polyester film of the present invention is characterized in that it has a light-diffusing layer (B) on one side of the intermediate layer (A) and a smooth layer (C) on the opposite side, which is laminated by a co-extrusion method. The three-layer structure is formed, wherein the intermediate layer (A) is composed of a crystalline homopolyester or a crystalline polyester containing a copolymerization component, and the light-diffusing layer (B) has a melting point of from 550 to 99 parts by mass of 225 ～. a crystalline polyester containing a copolymerization component at 255 ° C and 1 to 50 parts by mass of an additive which is incompatible with the polyester, and the smooth layer (C) is composed of a copolymerizable component having a melting point of 225 to 255 ° C. Made of crystalline polyester. The technical significance of using such a special layer construction is detailed below.

(1) Light diffusion layer (B)

The light-diffusing polyester film of the present invention has a light-diffusing layer (B) composed of a crystalline polyester containing a copolymer component and an additive which is incompatible with the polyester. Here, the crystalline polyester means a polyester having a melting point. The melting point is the endothermic peak temperature at the time of melting detected by the so-called differential scanning calorimetry (DSC). When it is measured by a differential scanning calorimeter, it is included in the crystalline polyester if it is a polyester which observes a clear crystal melting heat peak as a melting point. Since the polyester constituting the light-diffusing layer (B) of the present invention has a crystal structure, the mechanical properties peculiar to the polyester film such as heat resistance, mechanical strength, and thickness precision are suitably maintained.

From the viewpoint of heat resistance, mechanical strength, and thickness accuracy of the film, many crystal structures are advantageous, and therefore, the higher the melting point of the polyester resin, the better. However, when the melting point of the polyester resin is high, the elongation stress accompanying the stretching increases, so if there is a non-compatible additive in the resin, voids (voids) are likely to occur, and the total light transmittance is lowered. Therefore, in order to suppress the occurrence of voids while maintaining the mechanical properties of the polyester, it is preferable to control the melting point of the resin constituting the light-diffusing layer (B) within a certain range. The lower limit of the melting point of the crystalline polyester constituting the copolymerizable component of the light-diffusing layer (B) is preferably 225 ° C, more preferably 230 ° C, and particularly preferably 235 ° C. When the melting point is 225 ° C or higher, the desired heat resistance, mechanical strength, and thickness accuracy can be suitably exhibited. Further, the upper limit of the melting point of the crystalline polyester constituting the copolymerizable component of the light-diffusing layer (B) is preferably 255 ° C, more preferably 250 ° C, still more preferably 245 ° C. When the melting point is 255 ° C or lower, the occurrence of voids in the light-diffusing layer (B) can be suitably suppressed.

The melting point of the crystalline polyester constituting the light-diffusing layer (B) can be controlled by introducing a copolymerization component. By introducing a copolymerization component into the polyester, generation of voids can be suppressed, and light transmittance and light diffusibility can be highly coexistent. However, if the copolymerization component is introduced excessively, the melting point of the polyester is lowered, and the original excellent characteristics of the biaxially stretched polyester film cannot be obtained, so care must be taken. The amount of the copolymerization component to be introduced is preferably 3 mol% or more, more preferably 5 mol% or more, and particularly preferably 8 mol% or more, based on the entire aromatic dicarboxylic acid component or the entire diol component. When the content of the copolymerization component is more than 3 mol%, the occurrence of voids can be suppressed, and it is easy to make the light transmittance and the light diffusibility highly coexist. On the other hand, the upper limit of the amount of introduction of the copolymerization component is preferably 20 mol% or less, more preferably 18 mol% or less, and particularly preferably 15 mol% or less, of the above component. When the content of the copolymerization component is 20 mol% or less, the melting point of the mechanical properties of the biaxially stretched polyester film to a practical range can be obtained. Further, the composition of the copolymerization component which can be used in the present invention is as follows.

The light-diffusing layer of the present invention contains an additive which is incompatible with the polyester to achieve suitable light diffusibility. A suitable aspect of the light-diffusing layer of the present invention has an uneven shape due to an immiscible additive on the surface of the light-diffusing layer. The light incident on the light diffusion layer (exposed by the light diffusion layer) is refracted and diffused in an arbitrary direction by the unevenness imparted on the surface of the film, and exhibits surface light diffusibility. Therefore, the light diffusing film of the present invention preferably has an internal haze lower than the surface haze.

The light diffusion system in the light diffusion layer (B) can be divided into scattering due to the surface structure of the film and scattering due to the internal structure of the film. The scattering system can be evaluated as a surface haze, and the scattering system described later can be used as an internal haze evaluation. The light scattering due to the internal structure of the void or the like is accompanied by backscattering, so that a high total light transmittance cannot be obtained. On the other hand, the light scattering system due to the surface structure does not greatly reduce the total light transmittance, and high light diffusibility can be obtained. Further, the material which is incompatible with the polyester which can be used in the present invention is as follows.

The light-diffusing layer in the light-diffusing polyester film of the present invention is composed of 50 to 99 parts by mass of the above-mentioned crystalline polyester containing a copolymer component and 1 to 50 parts by mass of an additive which is incompatible with the polyester. Formulated with the composition. The preferred blending ratio of the two is a blend of 75 to 98 parts by mass of the polyester and 2 to 25 parts by mass of the additive, more preferably 80 to 97 parts by mass of the polyester and 3 to 20 parts by mass of the additive.

Further, when the mixing ratio of the above additives is less than 1 part by mass, the ability of the additive to form unevenness on the surface of the film is insufficient, and sufficient surface light diffusing performance cannot be obtained. On the other hand, when the mixing ratio of the additive exceeds 50 parts by mass, light scattering at the additive/polyester interface is increased while the elongation stress of the polyester is increased, and voids are easily generated around the additive. As a result, the internal haze of the light diffusion layer becomes large, and the total light transmittance tends to decrease.

The surface haze of the light-diffusing layer (B) tends to be higher as the surface unevenness increases. Therefore, the particle size of the additive of the light-diffusing layer (B) is preferably large. In order to obtain a particle diameter effective for surface haze, the lower limit of the thickness of the light-diffusing layer (B) is preferably 3 μm or more, more preferably 4 μm, and particularly preferably 5 μm.

On the other hand, if the thickness of the light-diffusing layer (B) is considerably higher than the particle diameter of the incompatible additive, it is difficult to form the surface uneven structure efficiently. Therefore, when the thickness of the light-diffusing layer (B) is increased, the formation of surface unevenness is reduced, and the surface haze is lowered. Further, as the thickness of the light diffusion layer (B) increases, the internal haze caused by the internal structure of the light diffusion layer (B) increases, and the total light transmittance decreases. In order to achieve high total light transmittance and light diffusibility, it is preferable to control the thickness of the light diffusion layer (B) within a specified range. Therefore, the upper limit of the thickness of the light-diffusing layer (B) is preferably 50 μm, more preferably 30 μm, and particularly preferably 20 μm.

(2) Smoothing layer (C)

A person who imparts moderate flexibility to the surface of the lens of the laminated film is suitable for lens processing. That is, when the hardness of the lens resin is increased or the lens shape is complicated, strong stress is generated on the surface of the substrate film. Further, when the productivity of the lens processing is improved and the base film is passed through the mold member of the roll mold at a high speed, there is a case where the base film is difficult to follow the mold member of the roll mold. Therefore, it is preferable to impart flexibility to the smooth layer (C) to which the lens layer is applied, since the stress relaxation on the surface of the film can be promoted and the adhesion of the lens layer can be improved.

The smoothing modulus (C) of the light-diffusing polyester film of the present invention is preferably less than 4.0 MPa, more preferably 3.8 MPa or less, still more preferably 3.7 MPa or less. When the tensile elastic modulus of the smoothing layer (C) is at most the above upper limit, flexibility suitable for lens processing is achieved. On the other hand, the tensile elastic modulus of the smooth layer (C) is preferably 2.0 MPa or more from the viewpoint of maintaining the mechanical strength of the surface of the film.

In order to control the tensile elastic modulus of the smoothing layer (C) within the above range, it is preferred to control the melting point of the resin constituting the smoothing layer (C). Therefore, the light-diffusing polyester film of the present invention preferably has a smooth layer (C) composed of a crystalline polyester containing a copolymerization component having a melting point of 225 to 255 °C. As described above, the higher the melting point of the polyester resin, the better the viewpoint of the mechanical properties of the film. However, in order to improve the softness of the surface of the film to which the lens layer is applied, it is preferred to control the melting point of the resin constituting the smooth layer (C) within a certain range. The lower limit of the melting point of the crystalline polyester containing the copolymerization component constituting the smoothing layer (C) is preferably 225 ° C, more preferably 230 ° C, still more preferably 235 ° C, and particularly preferably 240 ° C. When the melting point is 225 ° C or higher, the heat resistance, mechanical strength, and thickness accuracy which are desired as the base film can be suitably exhibited. Further, the upper limit of the melting point of the crystalline polyester containing the copolymerization component constituting the smoothing layer (C) is preferably 255 ° C, more preferably 250 ° C. When the melting point is 255 ° C or lower, the smooth layer (C) exhibits appropriate flexibility when the lens layer is applied, and appropriate lens adhesion can be achieved.

The melting point of the crystalline polyester constituting the smooth layer (C) can be controlled by introducing a copolymerization component. The amount of the copolymerization component to be introduced is preferably 3 mol% or more, more preferably 5 mol% or more, and particularly preferably 8 mol% or more, based on the entire aromatic dicarboxylic acid component or the entire diol component. When the content of the copolymerization component is more than 3 mol%, the flexibility of the surface of the film during lens application is improved, and lens processing is easy. On the other hand, the upper limit of the amount of introduction of the copolymerization component is preferably 20 mol% or less, more preferably 18 mol% or less, and particularly preferably 15 mol% or less, of the above component. When the content of the copolymerization component is 20 mol% or less, the melting point of the mechanical properties of the biaxially stretched polyester film to a practical range can be obtained. Further, the composition of the copolymerization component which can be used in the present invention is as follows.

The surface of the smoothing layer (C) is preferably a smooth surface so as to be easily applied to processing such as lens processing or hard coating processing. Specifically, the three-dimensional surface roughness (SRa) of the smoothing layer (C) is preferably 0.02 μm or less, more preferably 0.01 μm or less.

(3) Middle layer (A)

The light-diffusing polyester film of the present invention has a three-layer structure as described above, and both of the surface layers are made of a crystalline polyester containing a copolymerization component. Further, the light-diffusing polyester film of the present invention has an intermediate layer (A) composed of a crystalline homopolyester or a crystalline polyester containing a copolymerization component. The intermediate layer (A) mainly has an effect of imparting mechanical strength to the film. In other words, the rigidity of the base film (A) is imparted to the intermediate layer (A), and the task of smoothing the layer (C) is shared, and the flexibility and the feeling of stiffness can be coexisted.

From the viewpoint of mechanical properties, the crystalline homopolyester/crystalline polyester constituting the intermediate layer (A) preferably has a high melting point. Specifically, the lower limit of the melting point is preferably 250 ° C or higher, more preferably 255 ° C or higher. When the melting point is 250 ° C or more, suitable mechanical properties as a support layer can be achieved. The upper limit of the melting point is considered to be the upper limit of about 260 ° C from the viewpoint of the properties of the polyester.

(4) Layer structure

It is assumed that when the heating temperature rises as the productivity of the lens processing increases, or when the high-temperature heat treatment (for example, 100 ° C or more) is performed, the light-diffusing polyester film may be curled, which may occur in optical design or processability. problem. Therefore, the inventors of the present invention have focused on the thermal stress of each layer constituting the light-diffusing polyester film, and reviewed the laminated structure of the light-diffusing polyester film to adjust the linear expansion coefficient of each layer. With elastic modulus. As a result of the review, the light diffusion layer (B) and the smoothing layer (C) corresponding to each layer, particularly the outermost layer, are controlled by controlling the melting point and the layer thickness of the intermediate layer (A), the light diffusion layer (B), and the smoothing layer (C). The linear expansion coefficient and the elastic modulus are suitable for suppressing curl not only in the processing characteristics of the laminated body but also in the application of high-temperature processing.

That is, the amount of crimp of the laminate having a bimetallic structure is determined by the heating temperature and the modulus of elasticity of each layer and the coefficient of linear expansion. The physical property value acts as a shrinkage or expansion force (thermal stress), and when the thermal stress in the thickness direction is not uniform, the thermal stress acts by deforming and warping the planar shape to obtain a stable structure. Therefore, if the thermal stress in the thickness direction is controlled within an appropriate range, the curl can be controlled. Therefore, the inventors attempted to optimize the thermal stress of each layer obtained by heating. In other words, the laminated body is divided into two in the thickness direction, and the laminated structure is designed such that the thermal stress of the upper half and the thermal stress of the lower half become constant values at an assumed temperature.

The thermal stress is based on the linear elastic theory in composite mechanics and can be derived by the formulas (1) and (2).

σ _i = E _i α _i ( T ₂ - T ₁ )‧‧‧(2)

From the centerline of the central layer of the laminate film contained herein, [sigma] type heat stress, the number of division n-based laminate film, l _i based divided, σ _i based on the divided layers having a thermal stress, E _i based the elastic modulus of the divided layers, T ₁ before the heating temperature of the system, T ₂ based heating temperature, α _i based layer coefficients are divided by line until the time of the temperature change T ₂ T ₁ to expansion.

Furthermore, in the case of a three-layer laminated structure, not only thermal stress but also the distance between the center line and each layer and the degree of occurrence of stress at the distance point must be considered. That is, by converting the force into a moment, the difference due to the laminated structure can be eliminated. Therefore, in terms of the calculation method of the moment, it is preferable to subdivide the laminated structure so that the total torque value of the upper half corresponds to the total torque of the lower half. The unit of subdivision can be the smallest unit of thickness of each layer. For example, when a film of 100 μm laminated with 3 layers of 10 μm and 50 μm and 40 μm is used, since the minimum unit of the 3 layers is 10 μm, the thermal stress is separated from the center line by dividing into 10 μm×10 layers. By multiplying and summing up, the torque difference between the upper half and the lower half can be obtained. The difference in the moment between the upper half and the lower half thus obtained is the thermal stress. In order to suppress the occurrence of curl at the time of high-temperature heating of the light-diffusing polyester film of the present invention, it is preferred to reduce thermal stress. Specifically, the thermal stress at 150 ° C is preferably 0.7 N/m or less, more preferably 0.5 N/m or less, still more preferably 0.2 N/m or less, and particularly preferably 0 N/m. When the thermal stress at 150 ° C is in the above range, the curl due to the heat treatment at 150 ° C for 30 minutes can be appropriately made 3.0 mm or less.

As described above, in order to control the curl in order to reduce the torque difference, it is preferable to adjust the linear expansion coefficient and the tensile elastic modulus of the light diffusion layer (B) and the smooth layer (C) of the outermost layer to appropriate conditions. Specifically, the difference between the linear expansion coefficients of the light-diffusing layer (B) and the smoothing layer (C) is preferably 1.2 × 10 ^-5 / ° C or less, more preferably 1.0 × 10 ^-5 / ° C or less, and particularly preferably 0.5 or less. ×10 ^-5 /°C or less. Further, the difference between the tensile elastic modulus of the light-diffusing layer (B) and the smoothing layer (C) is preferably 1.0 MPa or less, more preferably 0.5 MPa or less. As long as a polyester resin is used, the linear expansion coefficient and the tensile elastic modulus are related to the melting point. Therefore, by controlling the melting point of each layer within the above range, the curl control at the time of heating can be appropriately performed.

That is, in order to highly control the occurrence of curl at high temperature treatment, it is preferable to highly control the melting point of the light diffusion layer (B) and the melting point of the smooth layer (C). For the light-diffusing layer (B), as described above, from the viewpoint of optical characteristics, in order to set a suitable melting point range, for the smoothing layer (C), in order to correspond to the melting point of the light-diffusing layer (B), it is preferable to have a melting point of 225. A crystalline polyester containing a copolymerization component at 255 ° C. When the melting point of the smoothing layer (C) exceeds the upper limit, curling tends to occur on the smooth layer (C) side, and when the melting point of the smoothing layer (C) is lower than the lower limit, curling tends to occur on the side of the light-diffusing layer (B). The lower limit of the melting point of the crystalline polyester containing the copolymerization component constituting the smoothing layer (C) is more preferably 230 ° C, particularly preferably 235 ° C, particularly preferably 240 ° C. Further, the upper limit of the melting point of the light diffusion layer is more preferably 250 °C. In order to control the melting point of the crystalline polyester constituting the smooth layer (C), it can be controlled by the addition amount of the copolymerization component as described above. Further, the difference between the melting point of the light-diffusing layer (B) and the melting point of the smooth layer (C) is preferably 10 ° C or lower, more preferably 8 ° C or lower, and particularly preferably 5 ° C or lower. When the difference in melting point is 10 ° C or less, the occurrence of curl at the time of heating can be more suitably suppressed.

Further, in order to more appropriately control the curl due to heating, it is also preferable to control the thickness of each layer. If the ratio of the light diffusion layer (B) to the entire thickness of the film is small, the additive in the light diffusion layer (B) may bleed out to the surface of the film or may fall off. On the other hand, if the ratio of the light diffusion layer (B) to the entire thickness of the film is large, the total light transmittance is lowered. Therefore, it is preferable to control the ratio of the light diffusion layer (B) to the entire thickness of the film to a specified range, preferably in the range of 2 to 50%. The lower limit of the ratio of the light diffusion layer (B) to the entire thickness of the film is preferably 2%, more preferably 3%, and particularly preferably 4%. On the other hand, the upper limit of the ratio of the light diffusion layer (B) to the total thickness of the film is preferably 50%, more preferably 35%, and particularly preferably 20%.

On the other hand, in the smoothing layer (C), from the viewpoint of controlling the curl due to heating, it is preferable to control the thickness of the layer corresponding to the light-diffusing layer (B). Specifically, the thickness of the smoothing layer (C) is preferably in the range of 5 to 50% of the total thickness, more preferably 10 to 40%, still more preferably 15 to 30%. When the thickness of the smoothing layer (C) is within the above range, it is preferable to more suitably reduce the curl.

The curl of the light-diffusing polyester film of the present invention by heat treatment at 150 ° C for 30 minutes is preferably 3.0 mm or less. The above curl is more preferably 2 mm or less, and particularly preferably 1.0 mm or less. When the curl measured by the above conditions is 3 mm or more, since the film is curled during post-processing at a high temperature, a high optical design cannot be maintained when the lens layer is provided.

Further, the structure and characteristics for obtaining the light-diffusing polyester film of the present invention are described in detail below.

(raw material)

The crystalline homopolyester used as a raw material of the film in the present invention is an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid or an ester thereof, and ethylene glycol or diethylene glycol. A polyester produced by polycondensation of a diol such as 1,3-propanediol, 1,4-butanediol or neopentyl glycol. These polyesters can be polycondensed by direct polymerization of an aromatic dicarboxylic acid directly with a diol, and by transesterification of an alkyl ester of an aromatic dicarboxylic acid with a diol. The transesterification method is produced by a method in which a diethylene glycol ester of an aromatic dicarboxylic acid is subjected to polycondensation or the like.

As a representative example of the foregoing polyester, polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate or polyethylene-2,6-polyethylene naphthalate. The polyester may be a homopolymer, and the third component may be copolymerized within a range that does not substantially impede the crystallinity. Among these polyesters, a polyethylene terephthalate unit or a 2,6-naphthalenedicarboxylate unit is preferably 70 mol% or more, preferably 80 mol% or more. The polyester is more preferably a polyester of 90 mol% or more.

Moreover, the crystalline polyester containing a copolymerization component which can be used in the present invention is a polyester having a third component (copolymerization component) introduced into the main chain by using the above-mentioned crystalline homopolyester as a basic skeleton, and has a structure and The molecular weight and composition are not limited and may be arbitrary.

Further, in the light-diffusing polyester film of the present invention, it is preferred to use an aromatic dicarboxylic acid component and ethylene glycol and a branched aliphatic diol or an alicyclic diol in a part or all of the raw material. a copolymerized polyester composed of at least one diol component.

The branched aliphatic diol may, for example, be neopentyl glycol, 1,2-propanediol or 1,2-butanediol. Further, examples of the alicyclic diol include 1,4-cyclohexanedimethanol, tricyclodecane dimethanol, and the like.

Among them, particularly preferred is neopentyl glycol or 1,4-cyclohexanedimethanol. Further, in the present invention, in addition to the above-described diol component, a combination of 1,3-propanediol or 1,4-butanediol is a more preferable embodiment. These diols are used as a copolymerization component, and are introduced into the above-mentioned range, and are suitable for imparting the above-mentioned characteristics, and further reducing the voids in the light-diffusing layer to make the light transmittance and light diffusibility highly high. It is also better to see the coexistence point.

Further, if necessary, one or two or more kinds of dicarboxylic acid components and/or diol components such as the following may be used in combination as the copolymerization component.

Examples of other dicarboxylic acid component which can be used together with terephthalic acid or an ester-forming derivative thereof include (1) isophthalic acid, 2,6-naphthalene dicarboxylic acid, and diphenyl-4. An aromatic dicarboxylic acid such as 4'-dicarboxylic acid, diphenoxyethane dicarboxylic acid, diphenylphosphonium dicarboxylic acid, 5-sodium sulfoisophthalic acid or phthalic acid or the like An ester-forming derivative, (2) an aliphatic dicarboxylic acid such as oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, fumaric acid or glutaric acid or an ester thereof a derivative, (3) an alicyclic dicarboxylic acid such as cyclohexanedicarboxylic acid or an ester-forming derivative thereof, or (4) an oxycarboxylic acid such as p-oxybenzoic acid or oxyhexanoic acid. Or their ester-forming derivatives and the like.

On the other hand, other diol components which can be used together with ethylene glycol and a branched aliphatic diol and/or an alicyclic diol, for example, an aliphatic group such as pentanediol or hexanediol An aromatic diol such as a diol, bisphenol A or bisphenol S, or an ethylene oxide adduct thereof, diethylene glycol, triethylene glycol or a dimer diol.

Further, if necessary, a polyfunctional compound such as trimellitic acid, trimesic acid or trimethylolpropane may be further copolymerized with the above polyester.

For the catalyst used in the production of the polyester, for example, an alkaline earth metal compound, a manganese compound, a cobalt compound, an aluminum compound, a ruthenium compound, a titanium compound, a titanium/ruthenium composite oxide, a ruthenium compound or the like can be used. Among these, from the viewpoint of catalytic activity, a titanium compound, a ruthenium compound, a ruthenium compound, and an aluminum compound are preferable.

In the production of the aforementioned polyester, it is more preferable to add a phosphorus compound as a thermal stabilizer. As the phosphorus compound, for example, phosphoric acid, phosphorous acid or the like is preferable.

The light-diffusing polyester film of the present invention can be directly used as a film raw material by using the above-mentioned copolymerized polyester, or a copolymerized polyester having a large copolymerization component and a homopolyester (for example, polyethylene terephthalate). Blending, adjusting the copolymerization component.

In particular, the film is produced by the use of the latter, and since it has the same light diffusibility and total light transmittance as when the copolymerized polyester is used, it has a high melting point (heat resistance) and can be adjusted to contain copolymerization. A crystalline polyester of the composition.

In addition, a method of introducing a third component (copolymerization component) into the main chain by melt-mixing two different kinds of crystalline polyesters and using the transesterification reaction of the two types may be employed. In particular, the aforementioned copolymerized polyester and at least one of polyethylene terephthalate and homopolyester other than polyethylene terephthalate (for example, polytetramethylene terephthalate or polyparaphenylene) The blending of butylene dicarboxylate) is preferable as a raw material of the light-diffusing polyester film of the present invention from the viewpoint of reducing voids.

Further, in the polyester constituting the intermediate layer (A), it is preferred that the polyester is substantially not contained. Further, in the crystalline copolymerized polyester constituting the light-diffusing layer (B), it is preferred that particles other than the additives described later are not substantially contained. The above-mentioned "substantially no particles", for example, in the case of inorganic particles, means that the inorganic element is quantified by fluorescence X-ray analysis to be 50 ppm or less, preferably 10 ppm or less, and particularly preferably not more than the detection limit. Thus, by using a clean polyester raw material free from impurities, the occurrence of optical defects of the liquid crystal display can be suppressed.

The lower limit of the intrinsic viscosity of the crystalline polyester is preferably 0.50 dl/g, more preferably 0.52 dl/g. When the intrinsic viscosity is less than 0.50 dl/g, when the foreign matter removing filter is provided in the melt production line, the discharge stability at the time of extrusion of the molten resin tends to be lowered. In addition, when the crystalline polyester is used as the light-diffusing layer (B), when the inherent viscosity of the crystalline polyester is increased, the shearing force of the melt-stirring is increased, so that the additive is finely granulated and cannot be obtained. An effective dispersion diameter to the extent that a good uneven structure is imparted on the surface of the light-diffusing layer (B). Therefore, the upper limit of the intrinsic viscosity of the crystalline polyester is preferably 0.61 dl/g, more preferably 0.59 dl/g. When the intrinsic viscosity exceeds 0.61 dl/g, the dispersion diameter of the above-mentioned additive in the polyester becomes small, and the light diffusibility tends to be lowered.

(Additive <surface unevenness imparting agent>)

The purpose of the additive in the present invention is to impart irregularities on the surface of the light-diffusing layer to exhibit surface light-diffusing properties. The above-mentioned additives are not particularly limited as long as they are incompatible with the polyester, and may be any, but it is preferred to use a material as described below.

(thermoplastic resin incompatible with polyester)

The most preferred additive usable in the present invention is a thermoplastic resin which is incompatible with the aforementioned polyester. That is, by utilizing the incompatibility between the polyester and the thermoplastic resin, in the process of the biaxially stretched film (melting/extrusion step), a thermoplastic resin incompatible with the polyester is formed in a matrix formed from the polyester. The field that has been formed is a technique used as a surface unevenness forming agent. Since the incompatible thermoplastic resin is easily deformed by heating, generation of voids in the inside of the film can be suppressed, and it is suitable for forming an uneven shape suitable for light diffusibility. Further, by using this technique, in the melting and pressing step of the film, foreign matter is filtered through a high-precision filter, and the cleanliness necessary for the film for liquid crystal display can be achieved.

On the other hand, when non-melting polymer particles or inorganic particles to be described later are used as an additive, the fineness of the mesh of the filter which can be used in the process of the film is limited, and it is difficult to remove foreign matter with high precision. Further, when polymer particles or inorganic particles are used, voids are likely to occur at the interface between the particles and the polyester, and it is difficult to highly coexist light diffusibility and total light transmittance.

The thermoplastic resin which is incompatible with the polyester which can be used as the above-mentioned additive is exemplified by the following materials. That is, polyethylene, polypropylene, polymethylpentene, various cyclic olefin polymers, polyolefins, polycarbonates, miscellaneous polystyrene, aligned polystyrene, homopolystyrene, etc. Acrylic resins such as styrene, polyamine, polyether, polyester decylamine, polyphenylene sulfide, polyphenylene ether, polyether ester, polyvinyl chloride, polymethacrylate, etc., and copolymerization based thereon a substance, or a mixture of such resins, and the like.

Among them, in order to produce a film having a high light transmittance, it is particularly preferable to use an amorphous transparent polymer. On the other hand, when a crystalline polymer is used as an additive, the crystalline polymer becomes cloudy, the internal haze of the film becomes large, and the light transmittance is lowered.

The amorphous transparent polymer usable in the present invention may, for example, be the following. That is, polystyrene (PS resin), acrylonitrile ‧ styrene copolymer (AS resin), methyl methacrylate ‧ styrene copolymer (MS resin), cyclic olefin polymer, methacrylic resin , PMMA, etc.

Among these, an amorphous transparent polymer having a surface tension close to that of the polymer selected from the polyester is more preferable from the viewpoint of the void reduction. Such an amorphous transparent polymer having a surface tension close to that of polyester is particularly preferably polystyrene (PS resin), PMMA or the like.

The present inventors have found that when the difference in melt viscosity between the crystalline polyester constituting the light-diffusing layer (B) and the incompatible thermoplastic resin is the same degree, the two components are easily dispersed, and the thermoplastic resin is finely granulated in the light. A good uneven structure is not obtained on the surface of the diffusion layer (B), and the surface haze is lowered. Therefore, in the present invention, the difference in melt viscosity between the crystalline polyester containing the copolymer component and the incompatible thermoplastic resin constituting the light-diffusing layer (B) is preferably large. The difference in melt viscosity is preferably 35 Pa ‧ or more, more preferably 40 Pa ‧ or more. When the difference in melt viscosity is 35 Pa·s or more, the additive in the polyester of the additive has a good distribution particle diameter, and it is more preferable to easily achieve light diffusibility.

(non-melting polymer particles)

The non-melting polymer particles which can be used as the additive of the present invention are not subjected to a melting point measuring device (MPA100 type manufactured by Stanford Research Systems Co., Ltd.), and are heated up to 350 ° C at 30 ° C at 10 ° C / min. The particle of the flow deformation due to melting is not limited in its composition. For example, an acrylic resin, a polystyrene resin, a polyolefin resin, a polyester resin, a polyamide resin, a polyimine resin, a fluorine resin, a urea resin, a melamine resin, and Organic polyfluorene-based resin or the like. The shape of the particles is preferably spherical or elliptical. Further, the particles may or may not have pores, and may be used in combination.

When the non-melting polymer particles are made of a polymer having a melting point of 350 ° C or higher, non-crosslinked polymer particles may be used, but from the viewpoint of heat resistance, it is preferred to use a crosslinked structure. Crosslinked polymer particles formed by the polymer.

The average particle diameter of the non-melting polymer particles is preferably from 0.1 to 50 μm. The lower limit of the average particle diameter of the non-melting polymer particles is more preferably 0.5 μm, particularly preferably 5 μm. In order to exhibit a good light diffusing effect, the average particle diameter of the non-melting polymer particles is preferably 0.1 μm or more.

On the other hand, the upper limit of the average particle diameter of the non-melting polymer particles is more preferably 30 μm, particularly preferably 20 μm. When the average particle diameter of the non-melting polymer particles exceeds 50 μm, the film strength or the total light transmittance is likely to be low. The non-melting polymer particles preferably use particles having a particle size distribution as narrow as possible.

The non-melting polymer particles may be one type or two or more types. In the case of using a plurality of non-melting polymer particles having a narrow particle size distribution (meaning that the particle diameter of the particles is uniform) and having a different average particle diameter, it is a preferred embodiment because coarse particles of the film can be prevented from being mixed.

Further, the measurement of the average particle diameter of the above particles was carried out by the following method.

A photograph of the particles was taken by a scanning electron microscope (SEM), and the maximum diameter of the particles of 300 to 500 was measured with a minimum particle size of 2 to 5 mm, and the average value thereof was regarded as an average particle diameter. Further, when the particles contained in the film were alone, the maximum diameter of each particle was measured, and the average value thereof was taken as the average particle diameter.

(inorganic particles)

Examples of the inorganic particles used for the additive include vermiculite, calcium carbonate, barium sulfate, calcium sulfate, alumina, kaolin, talc, and the like.

The average particle diameter of the above inorganic particles is usually preferably from 0.1 to 50 μm, more preferably from 0.5 to 30 μm, still more preferably from 1 to 20 μm. When the average particle diameter is less than 0.1 μm, a good light diffusion effect cannot be obtained. On the contrary, when it exceeds 50 μm, it is unfavorable because of a decrease in the strength of the film. The particle size distribution of the inorganic particles is preferably as narrow as possible. When it is necessary to expand the particle size distribution, it is preferred to match a plurality of particles having a narrow particle size distribution. By this correspondence, it is possible to suppress the incorporation of particles having a large particle diameter which is a defect of the film.

Further, the measurement of the average particle diameter of the above particles was carried out by the following method.

A photograph of the particles was taken by a scanning electron microscope (SEM), and the maximum diameter of the particles of 300 to 500 was measured with a minimum particle size of 2 to 5 mm, and the average value thereof was regarded as an average particle diameter. Further, the maximum diameter of the particles contained in the film was measured, and the average value thereof was taken as the average particle diameter.

The shape of the inorganic particles is not limited, and is preferably substantially spherical or true spherical. Further, the particles may be either non-porous or porous. Furthermore, both can be used together.

The additive used in the present invention may be one of the above three types, or two or more types may be used in combination.

[Characteristics of Light-Diffusing Polyester Film] (face matching coefficient)

The light-diffusing polyester film of the present invention preferably has a surface alignment coefficient (ΔP) of 0.08 to 0.16. The lower limit of the surface alignment coefficient (ΔP) is more preferably 0.09, and particularly preferably 0.10. On the other hand, the upper limit of the surface alignment coefficient (ΔP) is more preferably 0.15, particularly preferably 0.14.

When the surface alignment coefficient (?P) exceeds 0.16, the number or size of the voids occurring around the additive may increase depending on the kind of the additive to be used. Therefore, the internal scattering (internal haze) becomes large, and the total light transmittance is lowered.

On the other hand, when the surface alignment coefficient is 0.08 or more, the characteristics of the biaxially stretched film can be exhibited, and heat resistance, mechanical strength, thickness uniformity, and the like are good, and occurrence of heating curl can be suppressed.

The method of controlling the surface alignment coefficient within the above range is arbitrary, and can be controlled, for example, by adjusting the ratio of the copolymerization component in the above-mentioned crystalline polyester containing the copolymer component. When the ratio of the copolymerization component in the light diffusion layer or the smooth layer is increased, the surface alignment coefficient is lowered, and when the ratio of the copolymerization component is decreased, the surface alignment coefficient is increased. The preferred ratio of copolymerized components is as described above.

(optical properties)

Next, in the present invention, the surface haze is 5% or more, more preferably 10% or more, particularly preferably 15% or more, and the internal haze is preferably lower than the surface haze. The surface haze is derived from the characteristics of the surface unevenness of the light diffusion layer. Therefore, when light is emitted from the surface of the film, or when light is incident on the surface of the film, light is refracted by the unevenness of the surface of the light-diffusing layer, and the surface haze becomes high. Therefore, the surface haze is basically independent of the total light transmittance. Therefore, by increasing the surface haze, the light diffusibility can be improved while suppressing a decrease in the total light transmittance.

On the other hand, the internal haze is derived from the characteristics of light scattering inside the film. Therefore, the total light transmittance is lowered due to the backscattering of the incident light. Therefore, in order to produce a light diffusing polyester film having excellent light diffusibility and high total light transmittance, an effective means is to increase the surface haze while minimizing the internal haze. In order to suitably suppress the internal haze, it is preferred to use an incompatible thermoplastic resin as a light diffusing additive. The incompatible thermoplastic resin is easily deformed by heating, and it is possible to suitably suppress the occurrence of voids in the inside of the film which is a cause of internal haze.

The surface-diffusion of the light-diffusing polyester film of the present invention is preferably 5% or more, more preferably 10% or more, still more preferably 15% or more, and particularly preferably a lower limit of 20%. When the surface haze is at least the above lower limit, the printed pattern of the light guide plate or the lamp image of the cold cathode tube exhibits an effective diffusion effect, and the effective light diffusing performance as the light diffusing film can be easily obtained.

On the other hand, the upper limit of the surface haze is preferably 60%, the more preferred upper limit is 70%, and the more preferred upper limit is 80%. When the surface haze is 80% or less, the internal haze can be suppressed and the total light transmittance can be increased.

Also, the internal haze is preferably lower than the surface haze. The upper limit of the internal haze is preferably 40%, more preferably 30%, particularly preferably 20%, and particularly preferably 10%.

When the internal haze is the same as or exceeds the surface haze, the internal haze becomes the main function of the light diffusing function of the film, and light scattering occurs (with backscattering) inside the film, and the total light transmittance is greatly reduced. . On the other hand, the lower limit of the internal haze is preferably 1%. In a film having an internal haze of less than 1%, a sufficient surface haze is not obtained.

Further, the light-diffusing polyester film of the present invention preferably has a total light transmittance of 86% or more. A lower limit of light transmittance is 87%, and a lower limit is preferably 88%.

Further, the light diffusing property of the light diffusing film can be quantitatively evaluated by, for example, image sharpness. The image sharpness is an index indicating the sharpness when the light source such as a fluorescent lamp is viewed through a film, and is evaluated in accordance with the usual method for measuring the sharpness of the image according to JIS K 7105 "Test method for optical properties of plastics". Image sharpness. The smaller the sharpness of the image, the better the concealability, indicating excellent light diffusion performance.

The light-diffusing polyester film of the present invention can provide image sharpness of 50% or less in a transmission method having an optical comb width of 2 mm. A better upper limit of image sharpness is 40%, and a better upper limit is 20%. Further, the smaller the image sharpness, the better, but if the image sharpness is to be lowered as much as necessary, the internal haze is high and the total light transmittance is lowered. In the present invention, the lower limit of the image sharpness is preferably 1%, more preferably 3%.

The light emitted from the illuminant (cathode tube or light guide plate) passes through the light diffusing film and the lens sheet. At this time, the light diffused by the light diffusing film is fitted to the condensing angle of the lens which is mainly a 稜鏡 type lens provided in the lens sheet, and is emitted in the front direction. Therefore, in the light diffusing film, not only the diffusibility, but also when combined with the lens sheet, it is preferable to have an optical design capable of obtaining a predetermined front luminance.

When the light is diffused at a wide angle through the light diffusing film, the ratio of the amount of light collected by the lens sheet, the cymbal sheet, and the lens layer is small, so that the brightness of the transmitted light that is emitted toward the front side is low. On the other hand, when light is diffused at a narrow angle through the light diffusing film, the ratio of the amount of light that is absorbed by the lens sheet or the lens layer is increased, but the amount of the diffused component is small. Therefore, the light diffusibility of the transmitted light is lowered, and the concealing property by the diffusing film or the uniformity of the brightness in the entire irradiated surface is lowered. Therefore, it is preferable to highly achieve the coexistence of the brightness of the light-diffusing film alone when the lens sheet or the lens layer is combined.

In order to obtain a measure of the most suitable optical design of the film having the condensing angle of the lens sheet, a review of the film was carried out, and as a result, the inventors found that by controlling the average slope of the surface of the light diffusion layer (Δa), when the light is When the diffusion diffusion angle is combined with the lens sheet, a light diffusing film having an optical design capable of achieving excellent front luminance can be realized.

When the surface unevenness profile of the surface of the light diffusion layer was observed using a micromap, a fine uneven contour of the mountain shape was observed. Light is reflected by the inclined surface formed by the unevenness of the mountain type. When the slope is more than or equal to a specific angle, the light is efficiently condensed by the combination with the lens sheet to increase the brightness.

The average slope (Δa) referred to herein is obtained by the surface unevenness profile observed by the micrograph machine. The height (y) of the surface unevenness profile is measured at each specified pitch (x), and the height difference (y _{n -} y _n+1 ) of two consecutive measurement points is divided by the measured pitch interval (x). The slope is measured at a predetermined length in two directions orthogonal to the longitudinal direction (longitudinal direction of the film) and the transverse direction (width direction of the film), and is obtained by averaging the average inclination gradient (Δa). That is, the average slope gradient (Δa) represents an average slope (inclination) due to the uneven structure formed on the surface of the light diffusion layer. In the present case, the average slope (Δa) is a factor that governs the coexistence of light diffusion and brightness. This light diffusion is caused by the uneven structure on the surface of the light diffusion layer, which is achieved when combined with the lens sheet.

The average slope (Δa) of the surface of the light diffusion layer is preferably 0.03 or more. When the Δa is 0.03 or more, not only the light diffusibility required for the concealability of the cathode tube or the like can be achieved, but also a sufficient amount of luminance can be achieved when combined with the lens film. The lower limit of Δa is preferably 0.04 or more, and more preferably 0.05 or more. The upper limit of Δa is preferably 0.10 or less, more preferably 0.09 or less, and still more preferably 0.08 or less. When the Δa exceeds 0.10, depending on the lens sheet to be used, back reflection due to optical design and in-plane reflection occurs, and improvement in front luminance is not observed.

The composition of the non-coherent additive constituting the light-diffusing layer (B) contributes to the formation of effective irregularities in the stretching step. It is considered that this is because the incompatible additive is pressed out to the outside by the elongation stress generated inside the film in the stretching step to form an effective uneven structure.

However, even if the uneven structure is provided in the initial stage of the film forming step, the uneven structure is flattened in the subsequent film forming step, and the unevenness (Δa) having the average inclination gradient (Δa) at which the specified brightness can be generated cannot be maintained when combined with the lens sheet. structure. For example, in order to reduce voids inside the film, heat treatment at a high temperature of 235 to 250 ° C is applied in the heat treatment step. At this time, the crystalline polyester containing the copolymerization component constituting the light-diffusing layer (B) is softened by heat treatment at a high temperature, and the slope of the uneven structure formed in the stretching step is flattened.

Therefore, in order to achieve the method of the average slope (Δa) of the invention of the present invention, it is preferable to increase the difference between the melting point of the resin constituting the light-diffusing layer (B) and the heat treatment temperature. When the difference between the melting point of the resin constituting the light-diffusing layer (B) and the heat treatment temperature is small, the light-diffusing layer is softened in the heat treatment step, and as a result, the surface uneven structure having an average inclination gradient (Δa) which is excellent in brightness cannot be formed. . However, if the difference between the melting point of the resin constituting the light-diffusing layer (B) and the heat treatment temperature is increased, the heat treatment temperature is lowered, so that the heat shrinkage rate of the film is deteriorated. When the melting point of the resin constituting the diffusion layer (B) is increased, the voids generated around the incompatible resin contained in the light-diffusing layer (B) remain without being lost by heat treatment. A film in which a void has occurred is not suitable because the internal haze is increased and the total light transmittance is lowered. Therefore, it is preferable that the difference between the melting point of the light-diffusing layer (B) and the heat treatment temperature is controlled within a range of from 9 ° C to 25 ° C, more preferably from 11 ° C to 23 ° C, and even more preferably 13 ° C. Above 21 °C.

(mechanical properties)

Moreover, in the present invention, since the crystalline polyester is used as a raw material of the film, excellent heat resistance, mechanical strength, and excellent thickness precision of the biaxially stretched film can be obtained.

Regarding the heat resistance, the dimensional change rate at 150 ° C is preferably 3% or less in the transverse direction and the longitudinal direction, more preferably 2.5%, more preferably 2%, and particularly preferably 1.5%, particularly good. The upper limit is 1%. On the other hand, the dimensional change rate in the transverse direction and the longitudinal direction at 150 ° C is preferably small, and it is considered that 0% is the lower limit. When the dimensional change rate is 3% or less, dimensional change or planarity does not deteriorate in high-temperature processing or use in a high-temperature environment, and good planarity is maintained. As a result, the original purpose of the light diffusing film which makes the brightness of the light exit surface of the backlight unit uniform can be achieved. Further, the longitudinal direction in the present invention means the flow direction (winding direction) of the film at the time of film formation, and the transverse direction means a direction perpendicular thereto.

Further, the lower limit of the tensile strength of the film is preferably 100 MPa, more preferably 130 MPa, and particularly preferably 160 MPa. When the tensile strength is 100 MPa or more, the mechanical strength of the biaxially stretched film is exhibited, and problems such as cracking, cracking, bending, and cracking are less likely to occur in the film processing step.

Further, the lower limit of the tensile elongation of the film is preferably 100%, more preferably 120%, and particularly preferably 140%. When the tensile elongation is 100% or more, flexibility can be imparted to the film, and defects such as cracking, cracking, bending, and cracking are less likely to occur after the lens is applied or the step of winding or cutting.

Further, the light-diffusing polyester film of the present invention preferably has a thickness of not more than 5.0%. When the thickness of the film is not more than 5.0%, wrinkles or projections are less likely to occur when the film is wound on the roll, and planarity can be maintained. As a result, the brightness of the light exit surface of the backlight unit becomes uniform, and the original purpose of the light diffusing film can be achieved.

Further, the thickness of the light-diffusing polyester film of the present invention is not particularly limited, but is preferably in the range of 25 to 500 μm, more preferably in the range of 75 to 350 μm.

(Manufacture of biaxially stretched film)

In the present invention, as a method for satisfying the above characteristics, for example, the following production method is preferably used.

The mechanical or optical properties of the film can also be controlled by film forming conditions. When the elongation temperature of the film is increased, the elongation stress is lowered, so that the alignment coefficient is lowered, and the occurrence of voids is suppressed. Further, the surface unevenness due to the incompatible additive is also easily formed, and it is preferable to extend at a high temperature from the viewpoint of the coexistence of the total light transmittance and the light diffusibility. However, when the stretching temperature is increased, the thickness variation of the film is increased, thickness unevenness or the like occurs, and it is difficult to obtain the original mechanical properties of the film. In the light-diffusing polyester film of the present invention, in order to achieve excellent mechanical properties, coexistence of total light transmittance and light diffusibility, it is preferred to appropriately control film forming conditions which vary depending on resin characteristics or required characteristics, in particular, The temperature at which it extends. When the polyester resin is stretched to produce the light-diffusing polyester film of the present invention, the temperature at the time of the transverse stretching is preferably in the range of from 120 ° C to 160 ° C.

In the preferred method for producing a light-diffusing polyester film of the present invention, a crystalline polyester containing a copolymerization component as a raw material of the light-diffusing layer (B) will be described in detail with respect to the use of polyethylene terephthalate. A representative example of a pellet of an ester copolymer (hereinafter also simply referred to as polyester).

First, as a film raw material, the polyester and the polyester-incompatible thermoplastic resin are each dried by vacuum drying or hot air drying so that the moisture content becomes less than 100 ppm. Next, each raw material is metered and mixed, supplied to an extruder, and melt-pressed into a sheet form. Further, by using an electrostatic application method, a sheet in a molten state was brought into close contact with a metal rotating roll (cooling roll) whose surface temperature was controlled at 10 to 50 ° C to obtain an unstretched PET sheet. In the respective raw materials of the present invention, it is important that the non-compatible additive is used in advance by using an extruder to melt-mix all or a part of the base polymer and the incompatible additive, and use it as a preliminary kneading main pellet.

In this case, in order to control the occurrence of foreign matter such as a deteriorated product, it is preferable to control the temperature of the resin before the melting portion, the kneading portion, the polymer tube, the gear pump, and the filter of the extruder at 220 to 290 ° C. The resin temperature up to the polymer tube and the die was controlled at 210 to 295 °C.

Further, in a place where the molten resin has been maintained at a constant temperature of 275 ° C, high-precision filtration is performed to remove foreign matter contained in the resin. In the filter medium for high-precision filtration of the molten resin, the filter material of the stainless steel sintered body is excellent in the performance of the agglomerates containing Si, Ti, Sb, Ge, and Cu as a main component or a high-melting organic substance in the resin. When the temperature of the molten resin is lower than 275 ° C when the high-precision filtration is performed, the filtration pressure is increased, so that the discharge amount of the raw material resin or the like is reduced.

Further, the filter particle size (initial filtration efficiency: 95%) of the filter medium is preferably 20 μm or less, and particularly preferably 15 μm or less. When the filter particle size (initial filtration efficiency: 95%) of the filter medium exceeds 20 μm, it is difficult to sufficiently remove foreign matter having a size of 20 μm or more. By using a filter material having a filter particle size (initial filtration efficiency of 95%) of 20 μm or less to perform high-precision filtration of the molten resin, productivity is lowered, but optical defects due to coarse particles are few films. For important steps. Further, in the present invention, by using a thermoplastic resin which is incompatible with the crystalline copolymerized polyester as an additive, the above-described high-precision filtration system is possible.

In order to laminate the light-diffusing layer (B), the intermediate layer (A), and the smoothing layer (C) together, two or more extruders are used, and the raw materials of the respective layers are extruded, and a multilayer feeding head is used (for example). The merging head having the angular merging portion combines the layers, presses the sheet shape by the slit-shaped die, and cools and solidifies on the casting roll to produce an unstretched film. Alternatively, instead of a multi-layer feed head, a multi-channel die can be used.

Moreover, in the light-diffusing polyester film of the present invention, it is preferred to have a coating layer on at least one surface, and it is more preferable to have a coating layer on both surfaces. A preferred coating amount is in the range of 0.005 to 0.20 g/m ² . By providing the coating layer on the surface of the light-diffusing layer (B), the occurrence of reflected light on the surface of the film can be suppressed, and the total light transmittance can be further improved. Further, when a coating layer is provided on the smooth layer (C), and the surface of the coating layer is subjected to lens sheet processing or hard coating processing, easy adhesion can be imparted.

At this time, after the coating layer was provided on the unstretched film obtained by the above method, biaxial stretching was performed. The simultaneous biaxial stretching method or the sequential biaxial stretching method may be used. However, when the sequential stretching method is performed, an easy adhesion layer is disposed on the film extending through the longitudinal or transverse axis, and then extending in the orthogonal direction to perform the two axes. extend.

The method of applying the coating liquid for forming a coating layer onto the unstretched film or the one-axis stretched film may be selected from any known methods, and examples thereof include a reverse roll coating method, a gravure coating method, and a roll coating method. ), port mode coating method, roll brush method, spray method, air knife coating method, wire rod coating method, pipe doctor method, impregnation coating method, curtain coating method, etc., either alone or These methods are combined to coat.

The resin constituting the coating layer is preferably a copolymerized polyester resin or a polyurethane in terms of ensuring excellent adhesion to other optical functional layers in the use of a lens sheet or a light diffusing film. At least one or more of the resin or the acrylic resin is a main component. Further, these resins are also recommended from the viewpoint of suppressing the occurrence of reflected light on the surface of the light diffusion layer. In the resin constituting the coating layer, the above-mentioned "main component" means that at least one of the above resins is contained in an amount of 50% by mass or more based on 100% by mass of the resin constituting the coating layer.

In addition, in order to improve the transparency of the film, if the intermediate layer (A) does not contain particles or contains a small amount so as not to impede transparency, the slipperiness of the film is insufficient and the handleability is deteriorated. Therefore, in the above coating layer, for the purpose of imparting slipperiness, it is preferred to contain particles. Among these particles, in order to ensure transparency, it is important to use particles having an extremely small average particle diameter below the wavelength of visible light.

Examples of the particles include inorganic particles such as calcium carbonate, calcium phosphate, vermiculite, kaolin, talc, titanium oxide, aluminum oxide, barium sulfate, calcium fluoride, lithium fluoride, zeolite, and molybdenum sulfide, and the crosslinking is high. Molecular particles, organic particles such as calcium oxalate, and the like. When the coating layer is formed mainly from the above-mentioned copolymerized polyester resin, the vermiculite is particularly preferable. Since verteite has a refractive index close to that of polyester, it is most suitable for ensuring a light diffusing polyester film having excellent transparency.

From the viewpoint of ensuring transparency, handleability, and scratch resistance of the film, the particles contained in the coating layer preferably have an average particle diameter (the average maximum diameter of particles by the number of SEM observations) is 0.005. ~1.0μm. From the viewpoint of transparency, the upper limit of the average particle diameter of the particles is more preferably 0.5 μm, particularly preferably 0.2 μm. Further, from the viewpoint of handleability and scratch resistance, the lower limit of the average particle diameter of the particles is more preferably 0.01 μm, particularly preferably 0.03 μm.

Further, the measurement of the average particle diameter of the above particles was carried out by the following method.

A photograph of the particles was taken by a scanning electron microscope (SEM), and the maximum diameter of the particles of 300 to 500 was measured with a minimum particle size of 2 to 5 mm, and the average value thereof was regarded as an average particle diameter. In addition, when the average particle diameter of the particles contained in the coating layer is determined, a cross section of the coated film is taken to obtain a coating layer by using a transmission electron microscope (TEM) with a minimum particle size of 2 to 5 mm. The largest diameter of the particles present in the section. The average particle diameter of the particles formed by the aggregates was 300 to 500 shots at a magnification of 200 times using an optical microscope for the cross section of the coating layer of the coating film, and the maximum diameter thereof was measured.

The content of the particles in the coating layer is preferably from 0.1 to 60% by mass from the viewpoint of ensuring transparency, adhesion, handleability, and scratch resistance of the optical laminated film. . From the viewpoint of transparency and adhesion, the upper limit of the content of the particles is more preferably 50% by mass, particularly preferably 40% by mass. Further, from the viewpoint of handleability and scratch resistance, the lower limit of the content of the particles is more preferably 1% by mass, particularly preferably 0.5% by mass.

The above-mentioned particle system may be used in combination of two or more kinds, and the same kind of particles may be blended, but the particle diameters are different. The average particle diameter and the total content of the entire particles are preferably in the above range.

Next, the unstretched film obtained by the above method is simultaneously biaxially stretched or sequentially biaxially stretched, followed by heat treatment.

It is important that the above-described biaxial stretching is performed at a stretching ratio of 2.8 times or more in the longitudinal, lateral, and both directions. Further, the stretching ratio defined by the present invention means the actual stretching ratio of the actual stretching of the film. This stretching ratio can be grasped by the mass change rate per unit area before and after each stretching step, or by writing a grid-like magnification symbol of the unstretched film.

When the stretching ratio in any of the longitudinal direction or the transverse direction is less than 2.8 times, the thickness unevenness of the obtained film is deteriorated, and the excellent heat resistance and mechanical strength inherent to the biaxially stretched film are not obtained. Moreover, the thickness uniformity of the film is remarkably deteriorated. The lower limit of the preferred stretching ratio in the present invention is 3.0 times, and the more preferred lower limit is 3.2 times. Further, the preferred upper limit of the stretching ratio is 5 times.

When the polyester resin is stretched to produce the light-diffusing polyester film of the present invention, the temperature at the time of lateral stretching is preferably in the range of from 120 ° C to 160 ° C. Further, the heat treatment is preferably carried out in a temperature range of 220 to 250 ° C under conditions of an air volume of 25 m/min or more, and heat treatment is performed in the range of 5 seconds to 100 seconds.

When the elongation temperature of the film is increased, the elongation factor is lowered, the alignment coefficient is lowered, and the occurrence of voids is suppressed. Further, since the surface unevenness due to the incompatible additive is also easily formed, it is preferable to extend at a high temperature from the viewpoint of the coexistence of the total light transmittance and the light diffusibility. Further, when the heat treatment is performed at a high temperature, the voids disappear and the internal haze can be reduced. However, when the stretching temperature is increased, the thickness variation of the film is increased, thickness unevenness or the like occurs, and it is difficult to obtain the original mechanical properties of the film.

[Examples]

Next, the present invention will be specifically described using examples and comparative examples. First, the evaluation method of the characteristic value used in the present invention is shown below.

[Evaluation method] (1) Intrinsic viscosity

A mixed solvent of phenol (60% by mass) and 1,1,2,2-tetrachloroethane (40% by mass) was used as a solvent in accordance with JIS K 7367-5, and it was measured at 30 °C.

(2) Crystal melting heat, melting point and glass transition temperature

It was obtained using a DSC6220 differential scanning calorimeter manufactured by SII Nanotechnology Co., Ltd. The resin sample was heated and melted at 300 ° C for 5 minutes in a nitrogen atmosphere, and then quenched with liquid nitrogen, and 10 mg of the pulverized resin sample was heated at a rate of 20 ° C / min to carry out differential thermal analysis. The heat of crystal melting is obtained by integrating the DSC curves of the melting peak temperature (Tpm), the extrapolation melting start temperature (Tim), and the extrapolation melting end temperature (Tem) defined in JIS-K7121-1987 and 9.1. Further, the melting peak temperature (Tpm) is taken as the melting point. Further, the glass transition temperature (Tg) was determined based on JIS-K7121-1987 and item 9.3.

(3) Melt viscosity

The viscosity of the resin sample was measured in accordance with JIS K 7199 "Test method for the characteristics of plastic flow of a plastic capillary rheometer and a slit die rheometer", and Method A (capillary die) of 5.1.3. In the Toyo Seiki Capirograph 1B, using a capillary die of Φ1 mm and L/D=10, the dried resin sample was filled in a cylinder maintained at 270 ° C, and after melting for about 1 minute, at a shear rate of 608.0 sec. ^The melt viscosity was measured at ^-1 . In the case where a plurality of resins are used as the base polymer, the melt viscosity of the base polymer is sufficiently mixed in advance with a plurality of resin samples, and then filled in a cylinder, and the melt viscosity is measured by the same method as described above.

(4) Uneven thickness of the film

A continuous strip sample having a length of 3 m in the lateral direction and 5 cm in the longitudinal direction was wound, and the thickness of the film was measured by a film thickness continuous measuring machine (manufactured by Anritsu Co., Ltd.) and recorded on a recorder. The maximum value (dmax), the minimum value (dmin), and the average value (d) of the thickness were obtained from the graph, and the thickness unevenness (%) was calculated by the following formula. Further, when the length in the lateral direction is less than 3 m, bonding is performed. Furthermore, the joint portion is cut off by the above-described data analysis.

Uneven thickness (%) = ((dmax-dmin) / d) × 100

The measurement system was carried out three times, and the average value was obtained and evaluated by the following criteria.

○: The thickness is not less than 5%

×: thickness unevenness exceeds 5%

(5) Three-dimensional surface roughness (SRa) of the smooth layer (C) surface

For the surface of the smooth layer (C), a stylus-type three-dimensional space roughness meter (SE-3AK, manufactured by Kosaka Research Co., Ltd.) was used, and the length of the film was along the length of the needle at a radius of 2 μm and a load of 30 mg. The cutoff value of the direction was 0.25 mm, the measurement length was 1 mm, and the feed speed of the needle was 0.1 mm/sec. The measurement was carried out at a pitch of 2 μm to 500 points, and the height of each point was input into a three-dimensional space roughness analyzer (SPA-11). ). The same operation was carried out for 150 times in the width direction of the film at intervals of 2 μm, that is, 0.3 mm across the width direction of the film, and data was input to the analysis device. Next, using the analysis device, the center plane average roughness (SRa) was obtained.

(6) Haze, total light transmittance

The haze (昙 value) and the total light transmittance of the film test piece were measured in accordance with JIS K 7105 "Test method for optical properties of plastics". The film length of the film test piece was oriented in the vertical direction, and the light-diffusing layer (B) surface was set to face the light source side, and the measurement was performed using a NDH-300A type turbidimeter manufactured by Nippon Denshoku Industries Co., Ltd.

(7) Internal haze, full haze, surface haze

Applying cedarwood oil on both sides of the film test piece (coating amount: 20±10 g/m ² per side), high transparency polyethylene terephthalate with 2 halves of less than 1.0% haze A film (for example, A4300, manufactured by Toyobo Co., Ltd., thickness: 100 μm) was used as a sample for internal haze measurement. Further, two sheets of the highly transparent polyethylene terephthalate film were laminated with cedarwood oil as a blank sample.

Next, the haze of the sample for internal haze measurement and the blank sample was measured by the method described in (6). Then, the haze value of the blank sample was subtracted from the haze value of the internal haze measurement sample to determine the internal haze. Further, the haze of the film test piece alone measured by the method described in (6) was taken as the full haze, and the internal haze value was subtracted from the full haze value to determine the surface haze.

(8) Image sharpness

The measurement was carried out by a transmission method in accordance with JIS K 7105 "Test method for optical properties of plastics". The film test piece was measured in such a manner that the longitudinal direction of the film was perpendicular to the surface of the light-diffusing layer (B) toward the light source side. As the measuring instrument, an ICM-1T type image measuring instrument manufactured by SUGA Testing Machine Co., Ltd. was used.

(9) Tensile strength, tensile elongation

The measurement was carried out in accordance with JIS C 2318-1997 5.3.3 (tensile strength and elongation).

(10) Linear expansion coefficient

Separate film samples from each layer were taken. The linear expansion coefficient of each layer was measured in a measurement temperature range of 25 to 200 ° C using a thermomechanical analyzer (TMA SS-6100) manufactured by Seiko Corporation in accordance with JIS K 7197.

(11) Tensile modulus of elasticity

Separate film samples from each layer were taken. The tensile modulus of each layer was measured in accordance with JIS K 7161 using a tensile elongation analyzer (TMI RTM-100) manufactured by Toyo BALDWIN.

(12) Thermal stress

It is derived using the following formulas (1) and (2).

σ _i = E _i α _i ( T ₂ - T ₁ )‧‧‧(2)

From the centerline of the central layer of the laminate film contained herein, [sigma] type heat stress, the number of division n-based laminate film, l _i based divided, σ _i based segmented layer having a thermal stress, E _i Department The elastic modulus of the divided layer, T ₁ is the temperature before heating, T ₂ is the heating temperature, and α _i is the linear expansion coefficient when the temperature of the divided layer is changed from T ₁ to T ₂ . In the calculation, the following values are used. First, for T ₁ and T ₂ , in order to correspond to the crimp measurement method, T ₁ was 25 ° C and T ₂ was 150 ° C. Furthermore, E _i and α _i use the values obtained by (9) and (10). The n system is determined based on the minimum number of units when the three layers are subdivided. Furthermore, l _i is positive above the center line and negative under the center line.

(13) Size change rate

The measurement was carried out in accordance with JIS C 2318-1997 5.3.4 (dimension change).

(14) Surface alignment coefficient (ΔP)

The refractive index (nx) in the longitudinal direction of the film, the refractive index (ny) in the width direction, and the thickness direction are measured by an Abbe refractometer using a sodium D line as a light source by JIS K 7142-1996 5.1 (method A). The refractive index (nz) was calculated from the following equation (ΔP).

ΔP=(nx+ny)/2-nz

(15) Curl value

The crimp at the time of high temperature heating is measured by the following methods (a) to (f).

(a) A rectangular film sample having a film length of 300 mm in the longitudinal direction and 210 mm in the width direction was cut out.

(b) The light-diffusing layer (B) was placed on the surface, and the sample was placed on a flat hardboard paper, placed on a rack of a heating oven adjusted to 150 ° C, and heat-treated for 30 minutes.

(c) After the heat treatment, the sample was taken out together with the cardboard, and left at room temperature for 30 minutes. The room temperature conditions here are preferably under the management conditions of a temperature of 23 ± 2 ° C and a humidity of 65 ± 5%.

(d) The light-diffusing layer (B) was placed upward, and the sample placed over 30 minutes was placed on a horizontal glass plate, and the height of the curl of the four corners of the sample was measured with a JIS metal ruler (0.5 mm scale). (the height from the horizontal plane in the vertical direction), and visually observe 1/1 of the minimum scale. For all the samples, the curl heights of the four corners were measured, and the average of the curl heights of the four corners was obtained as the high temperature curl of the sample.

(e) On the other hand, when the curl height of the sample after standing at room temperature after heating is 0 mm or the cross section of the sample (either side of the rectangle) is M-shaped, the upper and lower surfaces of the sample are reversed (the light is made The diffusion layer (B) was measured downward, and the curl heights of the four corners were measured, and the average of the curl heights of the four corners was obtained as the high temperature curl of the sample.

(f) In the value of the high temperature curl measured by the above, a symbol is attached in order to clearly indicate the direction of the curl. That is, when the crimping is a concave portion on the surface side of the light-diffusing layer (B), a sign of + is attached to the value. On the other hand, when the crimping is a concave portion on the side of the intermediate layer (A), a sign of - is attached to the value.

(16) Lens adhesion

A lens-type transfer roller (diameter: 300 mm) having a dome angle of 65° and a pitch of 50 μm was injected and stretched to form an ultraviolet curable resin composition, and the film was coated at a speed of 2 m per minute, and the wavelength was from the film side. 200 ~ 600nm of the ultraviolet manner as to be 0.8J / cm ² irradiation high pressure mercury lamp, and a light diffusion layer on the smooth polyester film (C) face (Note that, Comparative Example B-based layer 2 and Comparative Example 3 a layer system A lens mold is formed thereon. The residual of the lens array was seen to be × on the lens mold, and ○ was not seen.

(17) Average slope gradient (△a)

The surface unevenness profile of the surface of the light-diffusing layer (B) was measured by using a three-dimensional shape measuring device (manufactured by Ryoden Co., Ltd., a micrograph machine TYPE 550, an objective lens 10 times). From the measured profile, the cross-sectional profile was cut out in two orthogonal axes in the longitudinal direction (longitudinal direction) and the lateral direction (width direction) of the film. For each direction, the height (y) was continuously measured at intervals of 1.0 mm and 2.5 μm, and the respective heights y ₁ , y ₂ , y ₃ ... y _n (μm) at intervals of 2.5 (μm) were output to Excel. In the file. The height of the protrusion is used for the output of Excel, and the Wave function of the analysis software (SX-Viewer, manufactured by R&D Systems, Inc.) is used. Furthermore, Δa is derived by calculating the following equation. Δa is an average slope gradient for each of the longitudinal direction and the lateral direction, and the values of the longitudinal and lateral directions are averaged.

Δa=[(y ₁ -0)/2.5+(y ₂ -y ₁ )/2.5+‧‧+(y _n -y _n-1 )/2.5]/n

(18) Brightness ratio

A lens sheet mounted on a liquid crystal television (Aquos LC-37GS10, manufactured by Sharp Corporation) manufactured by Sharp Corporation was used for the lens sheet to be combined for the evaluation of the brightness of the obtained surface light-diffusing film. The two films are laminated such that the intermediate layer (A) face of the cut light diffusing film sheet overlaps the lens back surface of the lens sheet. The light diffusing film (B) surface of the light diffusing film is placed on the turbidity meter so that the light diffusing film (B) surface of the light diffusing film is oriented in the vertical direction so that the longitudinal direction of the light diffusing film sheet is perpendicular. The turbidity meter was a turbidity meter of NDH-300A type manufactured by Nippon Denshoku Industries Co., Ltd. The measurement method was carried out in accordance with JIS K 7105 "Test method for optical properties of plastics". The value obtained by dividing the transmittance of the parallel line obtained by the measurement by the total light transmittance is derived, and the luminance ratio (%) of the value obtained by dividing the transmittance of the parallel line obtained by measuring the lens sheet alone by the total light transmittance is derived.

Example 1 (1) Manufacture of crystalline homopolyester resin (M1)

The temperature of the esterification reactor was raised, and at a time point of reaching 200 ° C, a slurry of terephthalic acid (86.4 parts by mass) and ethylene glycol (64.4 parts by mass) was added, and three as a catalyst were added while stirring. Cerium oxide (0.017 parts by mass) and triethylamine (0.16 parts by mass). Subsequently, the temperature was raised by pressurization, and the pressure esterification reaction was carried out under the conditions of a gauge pressure of 3.5 kgf/cm ² and 240 ° C. Then, the inside of the esterification reaction tank was returned to normal pressure, magnesium acetate tetrahydrate (0.071 parts by mass) was added, and phosphoric acid (0.014 parts by mass) was added. Further, the temperature was raised to 260 ° C for 15 minutes, phosphoric acid (0.012 parts by mass) was added, and sodium acetate (0.0036 parts by mass) was added. After 15 minutes, the obtained esterification reaction product was transferred to a polycondensation reaction tank, and the temperature was gradually raised from 260 ° C to 280 ° C under reduced pressure until the desired intrinsic viscosity, and the polycondensation reaction was carried out at 285 ° C.

After completion of the polycondensation reaction, filtration was carried out using a Naslon filter having a filter particle size of 5 μm (initial filtration efficiency: 95%), and the strands were pressed out from the nozzles, and the filtration was performed in advance (pore diameter: 1 μm or less). The water is cooled, solidified, and cut into pellets. The obtained crystalline polyester resin (M1) had a crystal heat of fusion of 35 mJ/mg, a melting point of 256 ° C, an intrinsic viscosity of 0.56 dl/g, and a melt viscosity of 91 Pa·s. Further, substantially no inert particles and internal precipitated particles are contained.

(2) Manufacture of copolymerized polyester resin (M2)

As a constituent component, 100 mol% of terephthalic acid unit as an aromatic dicarboxylic acid component, 70 mol% of ethylene glycol unit as a diol component, and 30 mol% of neopentyl glycol unit According to the production method of (M1), a copolymerized polyester resin (M2) having an intrinsic viscosity of 0.59 dl/g and a melt viscosity of 121 Pa‧s was produced.

(3) Polystyrene (M3)

A polystyrene resin (PS) having a melt viscosity of 147 Pa ‧ was used.

(4) Modulation of coating liquid (M4)

The transesterification reaction and the polycondensation reaction were carried out by a usual method to prepare 46 mol% of terephthalic acid and 46 mol% of isophthalic acid as a dicarboxylic acid component (to the whole of the dicarboxylic acid component). And 8 mol% of sodium 5-sulfonate isophthalate, 50 mol% of ethylene glycol as a diol component (to the total of the diol component) and 50 mol% of neopentyl glycol A water-dispersible sulfonic acid metal salt-based copolymerized polyester resin composed of the composition. Next, 51.4 parts by mass of water, 38 parts by mass of isopropyl alcohol, 5 parts by mass of n-butyl cellosolve, and 0.06 parts by mass of a nonionic surfactant are mixed, and then heated and stirred. If 77 ° C is reached, 5 mass is added. The copolymerized polyester resin of the above-mentioned water-dispersible sulfonic acid metal salt group is further stirred until the resin is not agglomerated, and the aqueous resin dispersion liquid is cooled to room temperature to obtain a uniform water dispersion having a solid content concentration of 5.0% by mass. Copolymerized polyester resin liquid.

In addition, after dispersing 3 mass parts of aggregated vermiculite particles (Sylysia 310 manufactured by Fuji SILYSIA Co., Ltd.) in 50 parts by mass of water, 99.46 parts by mass of the above water-dispersible copolymerized polyester resin liquid is added. 0.54 parts by mass of an aqueous dispersion of Sylysia 310, 20 parts by mass of water was added thereto with stirring to obtain a coating liquid (M4).

(5) Manufacture of light diffusing polyester film

74 parts by mass of a crystalline homopolyester (M1) which was dried under reduced pressure (1 Torr) at 135 ° C for 6 hours, and 23 parts by mass of a copolymerized polyester (M2) which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours. 3 parts by mass of polystyrene (M3) was supplied to the extruder 2 as a raw material of the light-diffusing layer (B). Further, a crystalline homopolyester (M1) which was dried under reduced pressure (1 Torr) at 135 ° C for 6 hours was supplied as a raw material of the intermediate layer (A) to the extruder 1. Further, 74 parts by mass of a crystalline homopolyester (M1) which was dried under reduced pressure (1 Torr) at 135 ° C for 6 hours and 23 parts by mass of a copolymerized polyester which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours. (M2) is supplied to the extruder 3 as a raw material of the smoothing layer (C).

The set temperature before the melting section, the kneading section, the polymer tube, the gear pump, and the filter of each extruder was 275 ° C, and the set temperature of the polymer tube after the filter was 270 ° C, using a 3-layer flow head. Each of the raw materials supplied from the extruder 2, the extruder 3, and the extruder 1 is laminated and melted and extruded into a sheet shape by a nozzle.

Further, a gear pump of each layer was used to control the thickness ratio of the (B) layer, the (A) layer, and the (C) layer so as to be 11 to 78 to 11. Further, in the above filter, a filter material of a stainless steel sintered body (labeling filtration accuracy: 10 μm particles of 95% cut off) was used. Further, the temperature of the nozzle was controlled so that the temperature of the resin to be extruded became 275 °C.

The pressed resin was adhered to a cooling drum having a surface temperature of 30 ° C by an electrostatic application method to be cooled and solidified to obtain an unstretched film. At this time, the (C) layer is used to contact the surface of the cooling drum. Further, the speed at which the cooling drum pulled the unstretched film was 12 m/min.

The resulting unstretched film was heated to 79 ° C using a preheat roll and extended to 3.4 times in the flow direction between rolls having different peripheral speeds. At this time, the temperature of the film was monitored by an infrared radiation thermometer, and the heater temperature was controlled so that the maximum temperature of the film became 100 °C.

After the longitudinal stretching was completed, one of the obtained axially stretched films was cooled to 50 ° C, and then the coating liquid (M4) was applied to both surfaces of the film. The solution coating amount was controlled so that the final film thicknesses on both sides were both 0.08 g/m ² . Then, the coated surface is dried in a drying oven.

The two ends of the one-axis stretched film having the coating layer were grasped by a jig, guided to a tenter, preheated to 120 ° C, extended 2.5 times in the width direction at 135 ° C, and extended in the width direction at 140 ° C. The heat treatment was further carried out at 231 ° C for 10 seconds, and during the cooling to 60 ° C, a relaxation treatment of 3.3% in the width direction was carried out to obtain a light-diffusing polyester film having a total thickness of 188 μm.

Further, in order to measure the melting point and the intrinsic viscosity of the polyester of each layer, the discharge of the (B) layer and the (C) layer was temporarily stopped, and the unstretched film of the (A) layer alone was collected. Similarly, a separate unstretched film of the (B) layer and the (C) layer was collected.

(6) Characteristics of the film

The properties of the film obtained in Example 1 are shown in Table 1. As is apparent from Table 1, the light-diffusing polyester film obtained by the present invention has excellent heat resistance, mechanical strength, and thickness precision inherent to the biaxially stretched film. Moreover, the internal haze is small and has a high light transmittance. Further, it is understood that most of the full haze is imparted by the surface haze, and the light diffusibility is also excellent. Also, almost no occurrence of curl occurred at the time of high temperature treatment.

Example 2

In addition to 87 parts by mass of a crystalline polyester (M1) which was dried under reduced pressure (1 Torr) at 135 ° C for 6 hours, 13 parts by mass of a copolymerized polyester (M2) which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours. As a raw material of the smoothing layer (C), it is supplied to the extruder 3, and the thickness ratio of the (B) layer and the (A) layer to the (C) layer is controlled so as to be 11 to 67 to 22 in the width direction. After the stretching, the light-diffusing polyester film of Example 2 was produced in the same manner as in Example 1 except that the heat treatment was carried out at 222 ° C for 10 seconds.

The properties of the film obtained in Example 2 are shown in Table 1. As is clear from Table 1, the present Example 2 has the same excellent characteristics as those of the first embodiment.

Example 3

In addition to 51 parts by mass of a crystalline polyester (M1) dried at 135 ° C for 6 hours under reduced pressure (1 Torr), 46 parts by mass of a copolymerized polyester which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours ( M2), 3 parts by mass of polystyrene (M3), which is supplied as a raw material of the light-diffusing layer (B) to the extruder 2, and mixed 53 parts by mass of crystals which are dried under reduced pressure (1 Torr) at 135 ° C for 6 hours. The homopolyester (M1), 47 parts by mass of a copolymerized polyester (M2) which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours, and supplied as a raw material of the smooth layer (C) to the extruder 3, adjusted The cooling roller draws the speed of the unstretched film so that the film thickness after stretching is 100 μm, and the thickness ratio of the (B) layer to the (A) layer and the (C) layer is controlled so as to be 22 to 56 to 22, and the coating liquid is applied. (M4) is applied only to the layer (C), and is extended by 2.4 times in the width direction at 135 ° C, then 1.6 times in the width direction at 140 ° C, and heat-treated at 223 ° C for 17 seconds, and is cooled to 60 ° C. In the same manner as in the first embodiment, a light-diffusing polyester film of Example 3 having a thickness of 100 μm was produced in the same manner as in the first embodiment.

The properties of the film obtained in Example 3 are shown in Table 1. As is clear from Table 1, the present Example 3 has the same excellent characteristics as those of the first embodiment.

Example 4

In addition to mixing 69 parts by mass of a crystalline homopolyester (M1) dried at 135 ° C for 6 hours under reduced pressure (1 Torr), 21 parts by mass of a copolymerized polyester which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours ( M2), 10 parts by mass of polystyrene (M3), which is supplied as a raw material of the light-diffusing layer (B) to the extruder 2, and mixed with 67 parts by mass of crystallinity which is dried under reduced pressure (1 Torr) at 135 ° C for 6 hours. The homopolyester (M1), 33 parts by mass of a copolymerized polyester (M2) which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours, was supplied as a raw material of the smoothing layer (C) to the extruder 3 at 232 ° C. The heat treatment was carried out for 17 seconds, adjusted to a thickness of 100 μm, and the thickness ratio of the (B) layer and the (A) layer to the (C) layer was controlled so as to be 11 to 84 to 7, as in the case of Example 3. In the method, the light diffusing polyester film of Example 4 was produced.

The properties of the film obtained in Example 4 are shown in Table 1. As is clear from Table 1, the present Example 4 has the same excellent characteristics as those of the first embodiment.

Example 5

In addition to mixing 55 parts by mass of a crystalline homopolyester (M1) which was dried under reduced pressure (1 Torr) at 135 ° C for 6 hours, 38 parts by mass of a copolymerized polyester which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours ( M2), 7 parts by mass of polystyrene (M3), which is supplied to the extruder 2 as a raw material of the light-diffusing layer (B), and mixed with 60 parts by mass of crystallinity which is dried under reduced pressure (1 Torr) at 135 ° C for 6 hours. The homopolyester (M1), 40 parts by mass of a copolymerized polyester (M2) dried under reduced pressure (1 Torr) at 70 ° C for 12 hours, supplied as a raw material of the smooth layer (C) to the extruder 3, at 224 The light-diffusing polyester film of Example 5 was produced by the same method as that shown in Example 1 except that the heat treatment was carried out for 10 seconds at °C.

The properties of the film obtained in Example 5 are shown in Table 1. As is clear from Table 1, the present Example 5 has the same excellent characteristics as those of the first embodiment.

Example 6

In addition to mixing 63 parts by mass of a crystalline homopolyester (M1) which was dried under reduced pressure (1 Torr) at 135 ° C for 6 hours, 34 parts by mass of a copolymerized polyester which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours ( M2), 3 parts by mass of polystyrene (M3), which is supplied as a raw material of the light-diffusing layer (B) to the extruder 2, and 60 parts by mass of crystals which are dried under reduced pressure (1 Torr) at 135 ° C for 6 hours are mixed. The homopolyester (M1), 40 parts by mass of a copolymerized polyester (M2) dried under reduced pressure (1 Torr) at 70 ° C for 12 hours, as a raw material of the smooth layer (C), supplied to the extruder 3, longitudinal The stretching ratio was 3.3 times, and the heat treatment was performed at 240 ° C for 17 seconds after the horizontal stretching, and the relaxation treatment was performed at 1.3% in the width direction during the cooling to 60 ° C, and the same as in the first embodiment. In the method, a light diffusing polyester film of Example 6 was produced.

The properties of the film obtained in Example 6 are shown in Table 1. As is clear from Table 1, the present Example 6 has the same excellent characteristics as those of the first embodiment.

Example 7

In addition to mixing 82.5 parts by mass of a crystalline homopolyester (M1) dried at 135 ° C for 6 hours under reduced pressure (1 Torr), 17 parts by mass of a copolymerized polyester which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours ( M2), 1.5 parts by mass of acrylic-polystyrene crosslinked particles having a particle diameter of 4 μm, which are supplied as a raw material of the light diffusion layer (B) to the extruder 2, the (B) layer and the (A) layer and (C) The light-diffusing polyester film of Example 7 was produced in the same manner as in Example 4 except that the thickness ratio of the layer was 18 to 72 to 10.

The properties of the film obtained in this Example 7 are shown in Table 1. In the seventh embodiment, although the surface haze is low, it is practically inferior in terms of optical characteristics, but the same excellent characteristics as in the first embodiment are obtained at the point of curling.

Example 8

In addition to 78.7 parts by mass of a crystalline polyester (M1) which was dried under reduced pressure (1 Torr) at 135 ° C for 6 hours, 21 parts by mass of a copolymerized polyester (M2) which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours. 0.3 parts by mass of acrylic-polystyrene crosslinked particles having a particle diameter of 8 μm, which are supplied as a raw material of the light diffusion layer (B) to the extruder 2, the (B) layer and the (A) layer and the (C) layer. The light-diffusing polyester film of Example 8 was produced in the same manner as in Example 5 except that the thickness ratio was 9:83 to 6.

The characteristics of the film obtained in this Example 8 are shown in Table 1. In the eighth embodiment, although the full haze and the surface haze are low, the optical characteristics are practically poor, but the same excellent characteristics as in the first embodiment are obtained at the point of curling.

Example 9

In addition to mixing 62 parts by mass of the crystalline homopolyester (M1) dried at 135 ° C for 6 hours under reduced pressure (1 Torr), 37 parts by mass of a copolymerized polyester (M2) dried under reduced pressure (1 Torr) at 70 ° C for 12 hours. 1 part by mass of the vermiculite particles having a particle diameter of 1 μm, which is supplied to the extruder 2 as a raw material of the light-diffusing layer (B), and produced in the same manner as in the example 6 Light diffusing polyester film.

The properties of the film obtained in this Example 9 are shown in Table 1. In the ninth embodiment, the total light transmittance is low, and it is practically inferior in terms of optical characteristics. However, the same excellent characteristics as in the first embodiment are obtained at the point of curling.

Comparative example 2

In addition to 74 parts by mass of a crystalline polyester (M1) which was dried under reduced pressure (1 Torr) at 135 ° C for 6 hours, 23 parts by mass of a copolymerized polyester (M2) which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours. 10 parts by mass of polystyrene (M3), as a raw material of the light-diffusing layer (B), is supplied to the extruder 2, and a light-diffusing layer (B) is provided instead of the smoothing layer (C), (B) layer and The light-diffusing film of Comparative Example 2 was produced in the same manner as in Example 1 except that the thickness ratio of the layer A and the layer (C) was 11 to 78 to 11.

Comparative example 3

In addition to 74 parts by mass of a crystalline homopolyester (M1) which was dried under reduced pressure (1 Torr) at 135 ° C for 6 hours, 23 parts by mass of a copolymerized polyester which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours ( M2), 3 parts by mass of polystyrene (M3), as a raw material of the light-diffusing layer (B), supplied to the extruder 2, without the smooth layer (C), the thickness of the (B) layer and the (A) layer A light-diffusing film of Comparative Example 3 was produced by the same method as that shown in Example 1 except that the ratio was 11 to 89.

Reference example 1

In addition to mixing 58 parts by mass of a crystalline homopolyester (M1) which was dried under reduced pressure (1 Torr) at 135 ° C for 6 hours, 32 parts by mass of a copolymerized polyester which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours ( M2), 10 parts by mass of polystyrene (M3), as a raw material of the light-diffusing layer (B), supplied to the extruder 2, without providing the smoothing layer (C), the thickness of the (B) layer and the (A) layer The ratio was 11 to 89, and the heat treatment was carried out at 234 ° C for 17 seconds, and in the course of cooling to 60 ° C, a relaxation treatment of 3.3% in the width direction was carried out, and the same manner as in the first embodiment was carried out. Reference is made to the light diffusing film of Comparative Example 1.

Reference Comparative Example 2

In addition to 51 parts by mass of a crystalline polyester (M1) dried at 135 ° C for 6 hours under reduced pressure (1 Torr), 46 parts by mass of a copolymerized polyester which was dried under reduced pressure (1 Torr) at 70 ° C for 12 hours ( M2), 3 parts by mass of polystyrene (M3), as a raw material of the light-diffusing layer (B), is supplied to the extruder 2, and the speed at which the cooling roller pulls the unstretched film is adjusted so that the thickness of the film after stretching becomes 100 μm. The smoothing layer (C) is not provided, and the thickness ratio of the (B) layer and the (A) layer is controlled to 20 to 80, and after extending 2.4 times in the width direction at 135 ° C, the width direction is extended by 1.6 times at 140 ° C. Further, the film was heat-treated at 233 ° C for 17 seconds, and a 1.0% relaxation treatment was performed in the width direction in the process of cooling to 60 ° C. A reference comparison of a thickness of 100 μm was carried out in the same manner as in the first embodiment. The light diffusing film of Example 2.

Industrial utilization possibility

The light diffusing polyester film of the present invention can be used as a light diffusing film used for a backlight unit, an illumination device, or the like of a liquid crystal display. Further, a base film for a crepe sheet can also be used. Therefore, it is of great help to the industry.

Claims (17)

A light diffusing polyester film which is a light diffusing polyester film formed by a biaxially oriented polyester film and having a light diffusion layer (B) on one side of the intermediate layer (A) on the opposite side A three-layer structure having a smooth layer (C) laminated by a co-extrusion method, the intermediate layer (A) being a crystalline homopolyester having a melting point of 255 ° C or higher or a crystalline poly-containing polymerized component The light-diffusing layer (B) is composed of 50 to 99 parts by mass of a crystalline polyester having a copolymerization component having a melting point of 225 to 255 ° C and 1 to 50 parts by mass of a non-compatible with the polyester. The smooth layer (C) is formed of a crystalline polyester containing a copolymerization component having a melting point of 225 to 255 ° C. The light diffusing polyester film of claim 1 has a surface haze of 15% or more and an internal haze of less than the surface haze. The light-diffusing polyester film of claim 1 is characterized in that the dimensional change rate at 150 ° C is 3% or less in the longitudinal direction and the transverse direction, and the tensile strength is 100 MPa or more in the longitudinal direction and the transverse direction, and the tensile elongation is 100%. the above. The light-diffusing polyester film of claim 1, wherein the rectangular film sample having a length of 300 mm in the longitudinal direction and a width of 210 mm in the film is curled at four corners after heat treatment at 150 ° C for 30 minutes in a heating oven. The height is an average of 3.0 mm or less. For example, the light diffusing polyester film of claim 1 has a total light transmittance of 86% or more, and the image width of the comb width of 2 mm is 40%. the following. The light-diffusing polyester film of claim 1, wherein on the surface of the light-diffusing layer (B), a copolymerized polyester resin, a polyurethane is provided before the film is stretched and the alignment is completed. A coating layer containing at least one of an acid ester resin or an acrylic resin as a main component. The light diffusing polyester film of claim 1, wherein the light diffusing layer (B) and the smoothing layer (C) of the light diffusing polyester film have a copolymerized polyester A coating layer containing at least one of a resin, a polyurethane resin, or an acrylic resin as a main component. The light-diffusing polyester film of the first aspect of the invention, wherein the crystalline polyester constituting the light-diffusing layer (B) contains 3 mol% or more and 20 mol% or less of a copolymerization component, and constitutes the smooth layer (C). The crystalline polyester contains 3 parts by mole or more and 20% by mole or less of the copolymerization component. The light-diffusing polyester film of claim 1, wherein the light diffusion layer (B) and the smooth layer (C) have a melting point difference of 10 ° C or less. The light diffusing polyester film according to claim 1, wherein the light diffusion layer (B) has a thickness of 2 to 20% with respect to the entire film, and the thickness of the smooth layer (C) relative to the entire film is 5~. 50%. The light-diffusing polyester film of claim 1, wherein the smoothing layer (C) has a tensile modulus of 2.0 MPa or more and less than 4.0 MPa. The light-diffusing polyester film of claim 1, wherein a difference in linear expansion coefficient between the light-diffusing layer (B) and the smoothing layer (C) is 1.2 × 10 ^-5 /°C or less. The light diffusing polyester film of claim 1, wherein the smoothing layer (C) has a three-dimensional surface roughness (SRa) of 0.02 μm or less. The light diffusing polyester film of claim 1, wherein the intermediate layer (A) is polyethylene terephthalate. The light-diffusing polyester film of claim 1, wherein the additive which is incompatible with the polyester constituting the light-diffusing layer (B) is a thermoplastic resin which is incompatible with the polyester. A light-diffusing polyester film for a lens sheet having a copolymerized polyester on the surface of the smoothing layer (C) of the light-diffusing polyester film of any one of claims 1 to 15 A coating layer containing at least one of a resin, a polyurethane resin, or an acrylic resin as a main component. A lens sheet obtained by imparting a lens layer to a coating layer of a light diffusing polyester film for a lens sheet as claimed in claim 16 of the patent application.